Hey Jimsmith, some of the best spots are as mentioned Steyning, Friston and Stamner.
Friston is a good shout as you can go up there and find most of the good singletrack. There is a bike shop at the edge of seven sisters country park(Friston) and they have maps. I usually go past the shop up the hill and you’ll see a layby on the left. Park here, and just through the woods is the top of the DH run.
Stamner is great, lots of windy singletrack. I would park at one of the top carparks, off ditchling road, and go left from here and your at the top of lots of singletrack routes. This is probably the best place if you don’t want to get lost as you cant really stray too far, and there aren’t any big drops or anything for your inexperienced mate.
Steyning is my local spot and there are some ace tracks down off the south downs, hard to find though.
hope this helps. There is plenty of riding in the south!